In the field of semiconductor integrated circuits an electrical conductor pattern is often placed on a substrate. Such substrates may be integrated circuits themselves or may be packaging substrates on which one or more integrated circuit chips may be mounted. The conductor patterns will then provide electrical interconnections between various such circuit portions on a chip, between integrated circuits mounted on a single packaging substrate, and between such chips and external circuitry. The metalization used to form such a conductor pattern often will include chromium, either alone or in conjunction with other metals. A commonly used structure places a layer of chromium adjacent to the substrate, a layer of copper over the chromium, and a second layer of chromium over the copper. In such a structure, the first layer of chromium is used because it will adhere well to typical substrates. The layer of copper is provided for its superior electrical conductivity, and the second layer of chromium protects the copper and presents a less reactive exposed surface or acts as a solder dam.
Such metalized substrates are commonly prepared by applying a relatively uniform metalization layer on a major surface of the substrate. A layer of photo-resist is then applied over the metalization layer. The photo-resist is exposed and developed leaving a resist layer covering those portions of the metalization to be retained, but leaving those portions of the substrate to be removed exposed. The exposed portion of the metalization layer is then brought in contact with an etching solution. The etching solution will remove those portions of the metalization layer lying beneath the exposed surface. When the etching is complete, only those portions of the metalization lying beneath the developed photo-resist will be retained. These portions form the desired electrically conductive network.
Because chromium is more difficult to etch than many other metals, some problems arise when chromium is included in the metalization. Such etchants used to etch chromium also tend to attack some commonly used photo-resist materials. Therefore, the choice of resists may be severely limited if such an etchant is to be used. Another problem arising with some etchants lies in the fact that they only etch chromium effectively when it is in contact with copper. Such etchants will not work effectively if chromium is used alone, and may even incompletely etch chromium in the Cr/Cu/Cr structure described above. Some prior art etchants will tend to etch chromium rapidly in a lateral direction resulting in undercutting in the chromium layer. Furthermore, some of the etchants will etch copper at a higher rate than chromium, thereby aggravating the undercutting problem. Even those etchants which do not etch copper more rapidly than chromium will often etch copper to some extent. Therefore a method of selectively etching chromium is desirable.